Surface Friction

Students drag a wooden block over different surfaces using a spring scale, recording the steady pulling force to compare kinetic friction. Results are analyzed across at least three trials per surface and used to discuss variables, error sources, and friction concepts.

1. Set two tape marks 40 cm apart on the table to define the pulling distance.
2. Check and zero the spring scale according to your teacher’s directions.
3. Attach the scale hook to one end of the wooden block so the largest face stays in contact with the test surface.
4. Practice pulling smoothly so the scale reading stays approximately constant along the 40 cm path.
5. Ignore the brief higher reading needed to start the motion; record only the steady value once the block slides at a nearly constant speed.
6. For the assigned surface (for example, lab table, sandpaper, linen, dowels, wax paper, paper towels), perform three trials and record the constant pulling force for each.
7. Repeat the three-trial measurement for each additional surface.
8. Compute the average constant force for every surface tested.
9. Measure the block’s mass with a balance and record it for later calculations (weight and coefficient of kinetic friction).
10. Clean up any test materials and ensure the work area is dry and safe.

Surface Friction Lab - C. Stephen Murray:

📄 Friction Lab - www.newpaltz.k12.ny.us: https://www.newpaltz.k12.ny.us/cms/lib/NY01000611/Centricity/Domain/812/Friction%20Lab%202019%20Science%208.pdf

• Test the effect of added water on a porous surface such as paper towel and compare to the dry condition.
• Flip the block to change the contact area while keeping the same surface to investigate area dependence.
• Add small masses on top of the block to test how normal force affects the required pulling force.
• Replace steady pulls with constant-speed pulls verified by a metronome or tick marks to improve consistency.
• Use different grit numbers of sandpaper to see how surface roughness changes the measured force.

• Keep fingers clear of the spring scale hook and block edges to avoid pinches.
• Ensure the path is clear of obstacles and liquids; wipe spills immediately to prevent slips.
• Pull gently and in line with the surface to avoid snapping the spring scale.
• Do not exceed the spring scale’s maximum rated force; use an appropriate range.

• What is the independent variable in this investigation? (Surface type.)
• What is the dependent variable in this investigation? (Average constant pulling force for 40 cm, a proxy for kinetic friction.)
• Which surface produced the smallest average constant force, and by how much compared with the largest? (Use your data to name the surfaces and report the numerical difference.)
• Why should the initial peak force be excluded from the data set? (It reflects static friction; the lab focuses on kinetic friction at steady speed.)
• If water is added to a paper towel surface, how could friction increase instead of decrease? (Water can increase adhesion and capillary bridging between fibers and wood, raising effective shear resistance.)
• How would you compute the block’s weight from its measured mass? (\(W=mg\), showing units in newtons.)
• What are likely sources of random error and systematic error in this lab? (Random: reading fluctuations, hand steadiness. Systematic: scale not zeroed, angle of pull adding lift or extra normal force, inconsistent contact area.)